[WIP] another parametric trait attempt

[tonyhffong]

This is a continuation of our earlier attempt on traits. https://github.com/mauro3/Traits.jl/pull/4

[tonyhffong]

The istrait() function is not working well for parametric yet.

[mauro3]

Thanks for the update. Does this mean that #4 can be closed? Also, maybe have a brief look at #19, which is the work I'm doing at the moment.